1. Field of the Invention
This invention relates to a regenerative adsorption column and, more particularly to a combined heaterless pressure-swing and purge sweep adsorption apparatus and process for the combined and sequential purification and fractionation of air in a single multi-zone column.
2. Description of the Prior
Two types of known heaterless adsorption systems for producing a product gas from a mixed gas feed are the pressure-swing adsorption system and the purge-sweep adsorption system. The two systems differ primarily in the method by which their adsorbent beds are regenerated.
Pressure-swing adsorption, or PSA, is described in Skarstrom U.S. Pat. No. 2,944,627 and has become known as the Skarstrom cycle. The Skarstrom cycle operates between an elevated adsorption pressure and a lower desorption pressure and is an essentially isothermal process that uses the heat generated on adsorption at the elevated pressure to do the work of desorption at the reduced pressure, ambient or vacuum. In pressure-swing adsorption, a purge serves to transport the heat of adsorption into the contaminated region of the adsorbent bed and to adsorb from the bed the contaminant vapors released by evaporation. The purge is not cooled in the process so the quantity of purge required to adsorb the liberated contaminant vapors is minimized. Thus, in pressure-swing adsorption the difference in adsorbate loading is that obtained between the two different pressures at isothermal conditions. Short cycles and low throughput per cycle ensure conservation of heat. Full regeneration is ensured by maintaining the volume of purge gas at least equal to the volume of feed ga at their different pressures.
In a purge-sweep system the heat of adsorption is not conserved. On regeneration, the heat required to desorb is supplied by the purge, which lowers the purge gas temperature, and thus decreases the capacity of the purge to sweep away contaminants. To compensate for the diminished capacity of the purge to desorb the adsorbent, the purge flow rate is increased. Regeneration of the adsorbent in a purge-sweep system therefore requires significantly more purge gas than does regeneration in a PSA system.
Heaterless systems are used for a wide variety of gas separations, either to purify gases or to enrich them in selected components. Such separations include, for example, the dehydration of air, the removal of contaminants, such as carbon monoxide, carbon dioxide and the like, from air, and the enrichment of argon in air, nitrogen in air, and oxygen in air.
In the fractionation of air to produce oxygen and nitrogen, water and carbon dioxide are regarded as impurity components of the air feed and are thus advantageously removed from air prior to fractionation or enrichment. It is particularly desirable to fractionate dehydrated air due to the well known improvement in oxygen recovery based on the fractionation of dehydrated air as opposed to humid air. For example, use of dehydrated air can result in as high as 30% more oxygen recovery. It may also be desirable to remove various other contaminants often found in the air feed.
Various methods have been employed to pretreat compressed air feed prior to oxygen, or nitrogen, enrichment. For example, heaterless adsorbers such as those operating on the Skarstrom cycle, have been used to remove moisture and other contaminants from the air feed. The pretreated air may then be fractionated, for example, in another heaterless adsorber column with adsorbents capable of carrying out the desired separation. However, such pretreatment of the air feed is not completely satisfactory because it typically adds significant cost and complexity to the overall air enrichment system and reduces the overall efficiency of the fractionation/enrichment process due to the purge and energy consumption in the pretreatment process.
Another example of an air feedstock pretreatment process to remove moisture and other contaminants prior to fractionation is the use of a reversing heat exchanger in combination with a heaterless adsorption system, as described in U.S. Pat. No. 4,380,457. The air separation process there disclosed includes passing an air supply under pressure through a reversing heat exchanger to cool the air and deposit water in the form of ice to form cool dried air; contacting the cool dried air with an adsorbent bed to remove at least carbon dioxide to form a residue of cool carbon dioxide free air; further cooling the cool carbon dioxide free air; and rectifying or fractionating the further cooled air.
More recently, it has been attempted to combine both air feed drying and air fractionation into a single column. This is generally described in Armond et al. U.S. Pat. No. 4,168,149 and U.K. Patent Application No. GB2,171,927A. However, neither reference discloses the combined purification and fractionation of an air feed as in the present invention. For example, Armond et al. '149 discloses drying sections at the inlet ends of the beds of an adsorbent column, which may contain, for example, silica gel, activated alumina or 5A or another zeolite molecular sieve. The drying sections do not purify the air feed as the column of the present invention purifies the air feed prior to fractionation. Instead, the drying sections merely scavenge the final vestiges of moisture in the air upstream of the fractionation beds, a technique well-known to those skilled in the art.
More specifically, a pretreatment step to remove the bulk of the moisture from the air feed is necessary in both Armond '149 and the U.K. patent as is further apparent from the subsequently issued U.K. Patent Application No. 2,171,927A itself, on which Armond is a coinventor. U.K. application '927A is directed to a gas separation process which includes two adsorbent beds, each bed having a first desiccant layer capable of removing residual water vapor from the air feed subsequent to treatment of the compressed air in a heat exchanger to remove the bulk of the moisture and a second adsorbent layer capable of fractionating the air feed. The process thus includes as an essential step the pretreatment of the compressed air feed upon exit from the compressor and upstream of the adsorbent beds to remove most of the water vapor from the air. The desiccant layer thus merely scavenges any residual moisture that may remain in the air following pretreatment. Moreover, because the bulk of the water vapor is removed prior to passing through the desiccant sections of the column, neither reference discloses or teaches any recognition of the importance of controlling the advance of the heat front generated in the purification zone of a multi-zone adsorbent which can be used for the combined purification and fractionation of an air feed.
Thus, despite the efforts of the prior art, there still remains a need to provide a combined heaterless pressure-swing and purge sweep adsorption column which is capable of sequentially purifying a compressed air feed and fractionating the purified air solely within the column and which does not require pretreatment of the air feed to remove a significant portion of the moisture, or to remove other contaminants that may be present in the compressed air feed.
Accordingly, it is the principal object of this invention to provide a simplified combined heaterless pressure-swing and purge sweep adsorption system which combines both purification and fractionation of a compressed air feed into a single adsorbent column without the need for prior separate treatment of the air feed to remove moisture, or to remove other contaminants that may be present in the compressed air feed.
Another object of the present invention is to provide a method for selectively producing either oxygen or nitrogen in a combined heaterless pressure-swing and purge sweep adsorption apparatus which has at least one column having two adsorbent zones, and which is capable of sequentially purifying the air feed and fractionating the purified air solely within the column, without the necessity of removing water vapor or other contaminants from the air feed prior to its entering the adsorption column.
A further and more detailed object of this invention is to provide a method for selectively producing oxygen in a combined heaterless pressure-swing and purge sweep adsorption apparatus which has at least one column having two adsorbent zones, and which is capable of sequentially purifying air feed laden with contaminants such as chemical warfare agents or industrial gases or both by the removal therefrom of such contaminants and fractionating the purified air solely within the column, without the necessity of removing such contaminants from the air feed prior to its entering the adsorption column.
Another specific object of the invention is to reduce operating costs in a heaterless adsorption system for the purification and fractionation of air by reducing the energy required to operate a heaterless adsorption system, and to reduce the capital cost of equipment for such a system.
These and other objects and advantages of the present invention will be apparent from the detailed description of the invention. While the invention will be discussed in connection with the purification and fractionation of air to produce oxygen, it is not intended to be so limited. On the contrary, and solely by way of illustration, the invention may also be used to effectively and efficiently purify and fractionate air to produce nitrogen. Moreover, it will be appreciated that the present invention may also be used where a prior pretreatment step has been employed to remove one or more contaminants that are in the air feed, but where there remain in the air feed contaminants that have not yet been removed by a prior pretreatment. For example, where the air feed contains water and other contaminants, it may be desirable to subject the air feed to a pretreatment which will remove water, but which does not remove the other contaminants. Contaminant laden air feed may then be fed to the multi-zone column of the present invention for the sequential purification (i.e., removal of contaminant) and fractionation of the air solely within the multi-zone column of the present invention.